Various systems have been proposed for replacing the traditional towing of barges on a line, which systems fall into the general category of vessels consisting of a tug positioned behind the barge in a pushing mode. These systems fall into two main classes: non-rigid connection systems allowing a certain degree of freedom between the tug and the barge and rigid systems in which all relative motions between the tug and the barge are restrained allowing both to act as a single vessel unit in a seaway.
In the rigid systems, there are also two main classes: integrated constructions where a tug fits snugly into a deep notch in the stern of the barge and an integrated construction where the tug-barge combination consists of incorporating a twin-screw catamaran tug that fits closely over and around the especially contoured barge stern which is tapered and sloped to form a wedge. In the first instance, it has been observed that, although the integrated system is relatively successful, tugs, in some cases, have stability problems when operated independently and, in other cases, are too specialized for normal service. In the second instance, it has been found that the form of the tug hull is quite unsuitable for even occasional independent operation as a tug boat. It can therefore be concluded from the examination of prior devices that an integrated tug-barge vessel, in order to be economically successful must satisfy at least the following criteria:
(a) it must not be too dimensionally critical so that barges and tugs can be built at different locations and still fit satisfactorily; PA0 (b) as there is a likelihood that there will be more barges than tugs, the interfacing surfaces of the barge must be as simple as possible; PA0 (c) the shape of the tug must be suited for harbor duty; PA0 (d) the tug must have sufficient stability for good independent operation.
In addition to these main points, are the normal criteria of smooth integration to allow a good speed and fuel consumption, ability to engage and disengage with moderate trim differences and the ability to disengage rapidly in an emergency, the latter point being a considerable safety advantage particularly for chemical and tanker barges where fire and explosion are a threat.